Percentage Calculator and Rule of Three
Pick the question, type the numbers and get the answer with the working shown: a percentage of a number, what percent one number is of another, the percent increase or decrease, and direct or inverse rule of three.
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Result
Type the numbers to see the answer and the steps.
Results with up to 4 decimals. A percent change is measured from the old value: going from 80 to 92 is +15%, coming back from 92 to 80 is -13.04%.
The formulas behind each answer
X% of a number
Multiply the number by the percent and divide by 100. 15% of 240 = 240 × 15 ÷ 100 = 36.
What percent
Divide the part by the whole and multiply by 100. 45 of 180 = 0.25 = 25%.
Percent change
New minus old, divided by the old value, times 100. Positive is an increase, negative a decrease.
Rule of three
Direct: when one grows the other grows too, x = B × C ÷ A. Inverse: when one grows the other shrinks (workers and days), x = A × B ÷ C.
Formulas
- X% of Y
- Y × X ÷ 100
- X is what % of Y
- X ÷ Y × 100
- Change
- (new - old) ÷ old × 100
- Direct rule of three
- x = B × C ÷ A
- Inverse rule of three
- x = A × B ÷ C
Small percentages add up.
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Create your free accountHow the attention economy worksHow to use it
- Pick the question in "What do you want to know?".
- Type the numbers in the fields that appear. Decimals and negative numbers work.
- Read the answer and the line with every step, using your numbers.
- Copy the result or download the card.
Examples
15% of 240
240 × 15 ÷ 100 = 36. Handy for tips, discounts and commissions.
From 80 to 92
(92 - 80) ÷ 80 = 0.15: an increase of 15%.
Rule of three
If 3 kg cost 12.6, 5 kg cost 12.6 × 5 ÷ 3 = 21. Inverse: if 4 workers take 6 days, 3 workers take 4 × 6 ÷ 3 = 8 days.
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Questions
How do I calculate a percentage of a number?
Multiply the number by the percent and divide by 100. 15% of 240 is 240 × 15 ÷ 100 = 36.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100: 45 out of 180 is 45 ÷ 180 × 100 = 25%.
How is percent change calculated?
Subtract the old value from the new one, divide by the old value and multiply by 100. From 80 to 92 is +15%.
What is the rule of three?
A way to find a fourth number when three are known and two quantities are proportional. If 3 kg cost 12.6, 5 kg cost 21.
When is the rule of three inverse?
When one quantity grows and the other shrinks: more workers, fewer days; more speed, less time. Then x = A × B ÷ C.
Is anything saved?
No. The numbers stay in your browser.